1. Field of the Invention
The present invention relates to a power converter employing, for example, a self-arc-extinguishing semiconductor device and, more particularly, pertains to a power converter which may be referred to as a multimode static series compensator, provided with such functions as reactive power compensation, voltage regulation and power system fault protection.
2. Description of the Background Art
Consumers of electric power often connect capacitors for power factor improvement to their loads to reduce contract charges for electricity by improving the power factor at receiving ends. Most of the loads of each consumer contain a reactor of which presence causes a lagging power factor. This is why the power factor can be improved by inserting capacitors which serve to produce a leading power factor.
If a consumer stops operating a load to which a capacitor for power factor improvement is connected, however, the capacitor itself acts equivalently as a load for a utility power system. Consequently, there arises a need for an electrical power company to supply leading reactive power to the equivalent phase-advancing capacitive load from a generator.
One of measures that the electrical power company can take to overcome this problem is to introduce a var compensator like the one shown in FIG. 8 of Japanese Laid-open Patent Application No. 1999-41812, for instance. The var compensator includes a power conversion circuit capable of producing reactive power required. The var compensator can generate either leading reactive power or lagging reactive power. Thus, the electrical power company causes the var compensator to generate the leading reactive power when the consumer operates the load and to generate the lagging reactive power when the load is not operated so that the reactive power to be generated by the generator is suppressed.
Some consumers have sensitive loads and there are cases where the loads are run at reduced operation rates depending on the quality of received utility voltage. A consumer running a semiconductor production line, for example, has a problem that, if the received voltage fluctuates, the production line would totally shut down. In particular, such a consumer would incur substantial losses if an instantaneous voltage drop occurs due to a power system fault.
One of conventional measures that a consumer can take to cope with such voltage fluctuations would be to introduce an uninterruptible power supply like the one shown in FIG. 1 of Japanese Laid-open Patent Application No. 1992-117135, for instance. The uninterruptible power supply directly supplies power fed from the utility power system to a load under normal conditions. If a voltage fluctuation occurs in the utility power system, the uninterruptible power supply disconnects the load from the utility power system by operating a switch and supplies electric power from a built-in battery to the load through a power conversion circuit.
Another conventionally known apparatus usable for coping with voltage fluctuations is a dynamic voltage restorer (DVR) shown in FIG. 1A of U.S. Pat. No. 5,329,222 which compensates for fluctuations in utility supply voltage caused by a system fault, for instance, without disconnecting the load from the utility power system unlike the aforementioned uninterruptible power supply. The DVR corrects for deviations in the utility supply voltage caused by line disturbances by regulating output voltage of a transformer of which primary winding is series-connected to a utility transmission line by means of a power conversion unit.
The aforementioned var compensator can suppress the reactive power to be generated by the generator by a reactive power compensating function, enabling the electrical power company to reduce power transmission cost. However, the var compensator does not work as an uninterruptible power supply which is needed by consumers. This means that the var compensator can not compensate for voltage drops or sags caused by a system fault, for instance.
On the other hand, the conventional uninterruptible power supply can supply voltage to a load even in the event of an instantaneous drop of the utility supply voltage upon disconnecting the load from a faulty power system by means of a high-speed circuit breaker or a thyristor switch. In principle, the uninterruptible power supply must provide full power required by the load in the event of a power system failure to compensate for any kind of instantaneous voltage deviations. Specifically, the uninterruptible power supply is required to provide a 100% voltage whether the voltage drop is 10% or 50% of a fully rated utility supply voltage. For this reason, it is necessary for the uninterruptible power supply to incorporate a power conversion circuit and a battery having large capacities, resulting in a high cost of the uninterruptible power supply.
In the DVR of U.S. Pat. No. 5,329,222, the transformer series-connected to a utility power system is held in a short-circuited state by the power conversion unit as long as no deviations or fluctuations exist in the utility supply voltage. Consequently, as long as the utility supply voltage is maintained at a rated level, the transformer and the power conversion unit connected to a secondary winding of the transformer remain in a standby state until a system fault occurs, while causing energy losses. The value of a current flowing through the power conversion unit is equal to a value obtained by multiplying a power line current by the turns ratio of the transformer which is set to a value larger than 1, so that the current flowing through the power conversion unit has a large value. Therefore, a large energy loss occurs in a network connected to the secondary winding of the transformer.